As of 1500, Pietro was engaged in a relationship with Lucrezia Borgia, the daughter of Pope Alexander VI. However, due to the tendency of her brother's – Cesare Borgia's – jealous rages towards her lovers, Lucrezia and Pietro kept their relationship a secret. However, in order to allow him to visit her easily and unnoticed, Lucrezia gave Pietro a key to the side door of the heavily-guarded Castel Sant'Angelo.

On one of Pietro's visits, Lucrezia instructed a guard to spy on the two of them as Pietro kissed her and openly confessed his love for her. After Pietro left, Lucrezia then told the guard to inform her brother of their relationship, wishing to purposefully ire Cesare.

In 1503, Cesare sent his personal assassin Micheletto Corella to kill Pietro during the Passion play in the Colosseum, for which Pietro performed as Jesus Christ. Unbeknownst to Pietro, Micheletto had infiltrated the cast and planned to stab Pietro to death in an "accident," under the guise of his role.

During the Crucifixion act, the play was interrupted by the AssassinEzio Auditore da Firenze, who intended to kill Micheletto before he could assassinate Pietro. Despite this, Micheletto had poisoned the wine that Pietro drank earlier in the act.

The doctor examining Pietro

Foregoing his planned assassination, much to Micheletto's surprise, Ezio instead rushed Pietro away from the fight to the nearest doctor for treatment. After Pietro appeared to be recovering from the poison, Ezio demanded that he give him the key to the Castel Sant'Angelo. While Pietro protested that he was only a "poor actor", Ezio rebutted that Cesare knew of his and Lucrezia's affair, to which Pietro finally handed over the key; thus, fearing for his life, Pietro subsequently fled Rome.